A circuit card is the current state of the art for building assemblies of electronic devices including a plurality of integrated circuits (“chips”). These assemblies can be separated into multiple types: organic multilayer laminated printed wire board (“PWB”), low temperature co-fired ceramic (“LTCC”), and high temperature co-fired ceramic (“HTCC”). Using each of these technologies, circuit card assemblies have been fabricated.
In a superconducting supercomputer, many of the operating processing integrated circuits (“chips”) are cooled to about 4K, but certain of the memory chips instead have a much warmer operating temperature of about 77K. Providing cooling at 4K is a costly activity, so every effort is made in superconducting supercomputer design to reduce the thermal parasitic load. This includes placing the assembly in vacuum (no convection), use of coatings and multilayer insulation to reduce radiation, and limiting the conductive thermal load between the “hot side” and “cold side” of the entire assembly.
One known method of achieving the desired operating temperatures for a superconducting supercomputer while avoiding thermal parasitic load even involves completely isolating the different-temperature chips in separate containers (e.g., vacuum containers or Dewars). Processing tasks must then be conducted via a very large number of wires extending several feet between the “hot side” and “cold side” containers, which adds considerable expense to the device and also appreciably slows down processing speed due to the distance the signals must travel.
Commonly available circuit cards have a minimum substrate thickness of 0.030″ with a deposited metal layer of about 0.0014″ thickness providing the circuit traces interconnecting the chips carried by the circuit card. In order to properly connect all the chips at 4K and 77K, the substrate will likely need to be thicker than the “standard” 0.030″ thickness, but as thickness increases, the thermal conduction parasitic load increases. The next issue at cryogenic temperature is the thermal expansion mismatch. Because the metal and dielectric layers are on the same order of thickness, the stress induced by changing temperatures between the two sides of the circuit card may lead to circuit card damage or warping. Warping may cause two primary problems: separation from the heatsink and device damage (lead separation).